Question: Multiply.
When a number is next to a variable, it means they are multiplied. So, $({3}{y^5})({5}{y^4})$ is the same as $({3})({y^5})({5})({y^4})$. Now we can rearrange the factors because multiplication is commutative (a fancy way of saying that the order in which we multiply things doesn't matter). ${(3)(5)}{(y^5)(y^4)}$ Then simplify, and we're done! ${15}{y^9}$